Sounds to me like you may have forgotten to "Bottom Dead Center" (BDC) the piston after you shut the engine off last. Imagine the piston sleeve being slightly narrower at the top compared to the bottom. This is called "pinch" and is what gives your engine its compression. As the engine heats up, the top of the piston sleeve expands to the same diameter as the bottom because of the heat being generated. It is vitally important that as soon as you turn off the engine (at any time) that you find the lowest point of travel of your piston and center it there. Otherwise, as your piston sleeve begins to cool, the piston will block it from cooling back down to the narrow "pinch". And you will lose that pinch. Couple of things you can do.

1) take a large / wide flat head screwdriver and pry the flywheel till it turns.
2) get a heat gun and get the engine nice and hot till you're able to loosen the piston.
3) (if you don't have a heat gun) take the engine off the car, remove the carb, wrap it in foil and stick it in the oven at abou 175c for about 10 minutes or so. That should do the trick.

Remember, always bottom dead center your piston after every run. It dramatically extends the life of your engine.

Good luck